The present invention relates to switch-mode power supplies. More particularly, the present invention relates to an apparatus and method for providing overcurrent protection for switch-mode power supplies.
Most modern electronic devices employ switch-mode power supplies due to the high power efficiency and smaller physical size of these types of power supplies. Switch-mode power supplies are also advantageous in that these supplies can provide regulated output voltages having magnitudes higher or lower than the unregulated input supply, and also provide multiple outputs of differing voltage magnitudes.
When switch-mode power supplies are overloaded or short-circuited, excessively high currents can result in severe damage to the power supply components. Presently, switch-mode power supplies typically employ one of four known overcurrent protection schemes and concomitant circuitry. These schemes include the so-called pulse-by-pulse scheme, the hiccup current limit scheme, the foldback current limit scheme, and the latch off scheme.
The pulse-by-pulse scheme clamps the peak of the power supply output current when a sensed current magnitude exceeds a threshold current magnitude. The power supply output is inhibited by reducing the duty cycle of a power switching device. A drawback with this overcurrent protection scheme is the high tail current that continues circulating in the output filter of the power supply even after the duty cycle of the switching device is reduced. Thus the power supply components can be overheated and damaged.
The hiccup current limit scheme terminates power supply operation completely once an overcurrent condition is detected. After a predetermined time period, a restart of the power supply is attempted. Normal power supply operation is resumed if the overcurrent condition has cleared; however, if the overcurrent condition is still present, power supply operation is once again terminated. This scheme typically requires an unacceptably long time to recover once an overcurrent condition has cleared. Additionally, this scheme is not suitable for parallel operation of power supplies.
The foldback current current limit scheme causes the power supply maximum current limit to decrease with the power supply output voltage. In other words, when the power supply output voltage decreases due to an overload or short-circuit, the maximum current limit also decreases thus reducing the output current to a safe level. This scheme is difficult to implement in isolated switch-mode power supply designs, because the output voltage is not readily available to the current limiting circuitry, thus extra circuitry is required to implement this scheme. Another problem with this scheme is that the power supply can latch up under certain operating conditions.
The latch off protection scheme completely shuts down the power supply under an overcurrent condition. Normal operation can only be restored by cycling the input voltage (on/off) or toggling an ENABLE input. Thus, the power supply is not capable of automatically restarting.
Hence, there is a need in the art for an overcurrent protection scheme for switch-mode power supplies that effectively protects the power supply components when a sensed current magnitude exceeds a threshold current magnitude, and automatically attempts a restart of the power supply to normal operation, without the drawbacks of the known overcurrent protection schemes.